A glass is a material that when cooled from its heated liquid transforms to the solid state without forming crystals. Such non-crystallized materials are also called amorphous materials. For example, one of the better known amorphous materials is quartz, which can be used to form conventional window glass. Most metals crystallize when they are cooled from the liquid state at reasonable rates, which causes their atoms to be arranged into a highly regular spatial pattern or lattice. A metallic glass is one in which the individual metal atoms have settled into an essentially random arrangement. Metallic glasses are not transparent like quartz glasses and are often less brittle than window glass.
A number of simple metal alloys may also be processed to form a glass-like structure. Binary metal alloys near deep eutectic features of the corresponding binary phase diagrams may be prepared into a glassy structure on cooling from the liquid state at rates greater than 1000 degrees per second. These binary metallic glasses may possess different properties than crystalline metals. These different properties may be useful in certain applications.
Bulk metallic glass forming alloys are a group of multicomponent metallic alloys that exhibit exceptionally high resistance to crystallization in the undercooled liquid state. Compared with the rapidly quenched binary metallic glasses studied prior to 1990, these alloys can be vitrified at lower cooling rates, less than 10 degrees per second.
Many of the recently discovered bulk glass forming alloys can be broadly described as pseudo-ternary alloys of the form ETM1-x-yLTMxSMy. Typically the early transition metal couple, ETM, is a combination of elements from group IVB of the periodic table; e.g., Zr and Ti. The late transition metals, LTM, are typically combinations of the 3d transition metals from groups VIIIB and IB; e.g., Fe, Co, Ni, and Cu. The simple metal element, SM, is normally chosen groups from IIA or IIIA; e.g., Be, Mg or Al. However, the addition of a SM element is not a requirement for the formation of a bulk glass forming alloy. There are also bulk metallic glass forming alloys based on magnesium.
Examples of some of the composition manifolds that contain ideal bulk metallic forming compositions are as follows: Zrxe2x80x94Tixe2x80x94Cuxe2x80x94Nixe2x80x94Be, Zrxe2x80x94Nbxe2x80x94Cuxe2x80x94Nixe2x80x94Al, Tixe2x80x94Zrxe2x80x94Cuxe2x80x94Ni, and Mgxe2x80x94Yxe2x80x94Cuxe2x80x94Nixe2x80x94Li. Each of the chemical species and their combinations are chosen for a given alloy composition such that the alloy composition lies in a region with a low-lying liquid surface. Alloy compositions that exhibit a high glass forming ability are generally located in proximity to deep eutectic features in the multicomponent phase diagram. These materials, including the recently developed families of Zr-based bulk metallic glass alloys show great promise as engineering materials. However, as in many metallic glasses, specimens loaded in a state of uniaxial or plane stress fail catastrophically on one dominant shear band, thus limiting their global plasticity. Specimens loaded under constrained geometries (plane strain) fail in an elastic/perfectly-plastic manner by the generation of multiple shear bands. Multiple shear bands are observed when the catastrophic instability is avoided via mechanical constraint. This behavior under deformation has limited the application of bulk metallic glasses as engineering materials.
The present application teaches a new class of metallic glass materials that employ the previously unknown physical mechanism of shear band pattern formation. The occurrence of shear band pattern formation dramatically increases the plastic strain to failure, impact resistance, and toughness of the material.
To exploit this phenomenon, a metallic glass matrix is combined with a ductile metal or metal alloy phase. The metallic glasses of this type may be glassy matrix composites based on bulk glass forming compositions in any bulk metallic glass forming alloy system. Formation of these objects is carried out using standard powder metallurgy techniques, at temperatures that are below the melting point of the individual constituents. Combinations of powders comprised of bulk metallic glass forming particles and crystalline ductile metal or metal alloy phases are employed. To prepare a ductile metal/bulk metallic glass matrix composite material, mixtures of metal or metal alloy powders are mixed with the bulk metallic glass powders, followed by processing in the super cooled liquid region (xe2x80x9cSLRxe2x80x9d). The SLR is defined as the difference in temperature between the glass transition and crystallization temperatures of the glass matrix. This temperature interval is defined as xcex94T=(Txxe2x88x92Tg), where Tg and Tx are the glass transition, and crystallization temperatures, respectively, of the bulk metallic glass constituent which is used to prepare the consolidated powder product or composite, and with the geometry desired. The control of the relative volume fractions of the ductile metal or metal alloy particles and bulk metallic glass matrix is simply controlled by the initial the mixing ratio. The maximum properties allowed by shear band pattern formation upon mechanical deformation are readily controlled in composites prepared in this fashion. This method also allows for bulk metallic glass matrix particles which incorporate crystalline ductile metal phases, formed from the molten state in situ, with a possible further increase in properties. The length scales, or size ranges, associated with the ductile metal or metal alloy phases may be of significantly differing magnitudes. Hence, these differing scales may result in duplex, triplex, or higher order multiplex morphological structures for the added particle sizes; each with a specific purpose. Namely, there will be a preferred size range, of the order of microns in which shear band pattern formation is encouraged. The particles added with larger length scales will further toughen the composite material formed by use of traditional composite toughening mechanisms such as, crack bridging, fiber pull-out, etc. The formation of shear band patterns through the material may cause new effects that had not been previously known in the art.
The present invention describes a material formed by a specified combination of ductile metal and bulk metallic glass matrix. More specifically, the system describes crystalline ductile metal particles being existing within a matrix of amorphous bulk metallic glass. Specific materials are described herein, but it should be understood that other materials may be used and other formation techniques. The system operates to toughen bulk metallic glasses using included ductile phases in a composite comprised of a metallic glass matrix.
For introductory purposes only, consider an embodiment for disclosure of the example of shear band pattern forming observed via in situ precipitation from the liquid state in the Zrxe2x80x94Tixe2x80x94Cuxe2x80x94Nixe2x80x94Be alloy system. The bulk glass forming compositions in the Zrxe2x80x94Tixe2x80x94Cuxe2x80x94Nixe2x80x94Be system are compactly written in terms of a pseudo-ternary Zrxe2x80x94Tixe2x80x94X phase diagram, where X represents the moiety Be9Cu5Ni4. Results have been obtained for alloys of the form (Zr100-x-zTix Mz)100-yXy, where M is an element that stabilizes a crystalline beta-phase in Ti- or Zr-based alloys. The composition of specific interest is (Zr75Ti18.34Nb6.66)75X25; i.e., an alloy with M=Nb, z=6.66, x=18.34, and y=25. Upon cooling from the high temperature melt, the alloy undergoes partial crystallization by nucleation and subsequent dendritic growth of the beta-phase in the remaining liquid. The remaining liquid subsequently freezes to the glassy state. This produces a two-phase microstructure containing beta-phase dendrites in a glass matrix.
The inherent properties of the final material impose constraints on the glassy matrix. Upon deformation these constraints lead to the generation of highly organized shear band patterns throughout the material. In the deformed regions of the material regularly spaced shear bands are seen where the spacing is coherent with the microstructural length scale. The patterns formed exist within domains that are dependent on the local orientation of the crystalline phase, and may have a spatial range extending up to 100 microns. Within each domain, regular parallel arrays of shear bands are observed at a spacing of typically 2 to 10 microns. This spacing may coincide with the secondary arm spacing of the beta-phase dendrites. Individual shear bands may occur, and may propagate through the ductile dendrites as highly localized twins.
The materials obtained may have a plastic strain to failure of up to or greater than 20 percent under unconfined loading conditions.
The initiation and propagation of the shear bands may be controlled by the scale and geometry of the ductile phase dispersion. The result is that deformation occurs through the development of highly organized patterns of regularly spaced shear bands that are distributed uniformly throughout the sample.
A monolithic bulk metallic glass object may be prepared from bulk metallic glass forming powders. These bulk metallic glass forming powders could be prepared via mechanical alloying (ball milling), rotary or centifugal atomization, gas or spray atomization, rotating anode, and/or sol-gel processes to name a few examples. The prior art in this area is extensive. This technique uses conventional powder metallurgy processing techniques, such as extrusion, hot-pressing, forging, rolling, and drawing to compact objects from the constituent powders. There are certain advantages to this technique. The compacted powder only requires heating to a relatively low temperature since consolidation of the powder is carried out in the supercooled liquid region or SLR.
In the Zr-based bulk metallic glasses, these operations are typically carried out around 300 to 400 degrees Celsius or 573 to 673 Kelvin (K). For an ideal system, the width of the supercooled liquid region should be relatively wide; e.g. 100 degrees Kelvin (K), in order to facilitate powder metallurgy processing techniques. Certain materials such as Zr-based alloys may facilitate formation in this region. This technique may also be applied to aluminum- and iron-based bulk metallic glass alloy systems. In all of said systems, once the object is formed, it should be cooled sufficiently rapidly so as to retain the metallic glass condition.
A bulk metallic glass matrix composite object that exhibits shear band pattern formation may also be formed by mixing of ductile metal or metal alloy powders with bulk metallic glass powders followed by compaction using powder metallurgy techniques. Specified metals or metal alloy powders are mixed with bulk metallic glass powders. Processing is again carried out in the supercooled liquid region to prepare the consolidated powder product or composite, having the desired geometry. The materials could be extruded under vacuum in an appropriate canister, such as copper, at pressures of the order 100 Mega Pascals (Mpa). The processing temperature could be reduced by using higher compaction pressures. The relative volume fractions of the materials are controlled by controlling an initial mixing ratio of ductile metal to bulk metallic glass. The control of the degree of shear band pattern formation upon mechanical deformation therefore may also be controlled. Since bulk powders are used, it may be easier to provide specified tailored microstructural properties based on different ratios between the ductile metal in the bulk metallic glass matrix material. Consider the following examples.